Inasmuch as Brioche goes into a hot oven in a room temperature loaf pan rather than a preheated pan, I don't see why your cast iron pot wouldn't work. But cast iron, while it retains heat wonderfully, won't heat quite as quickly as your typical loaf pan (which has thinner walls) so it may take a bit longer to bake.

You have to remember that the yeast will live to feed until the temperature reaches 140F. So gas production for an extended period due to the insulating properties of the heavy pan will allow the yeast to have a feeding frenzy for a longer period of time.

Explosive Brioche may be the result...by hey...try it!

If you don't have a steel pan...try a steel coffee can...it makes perfectly round loaves and the slicing is really cool for different fillings.